improve parallel build instructions

[leighmcculloch]

Description

Add an explicit example of how to use make -j to run a parallel
builds, i.e. make -j<N>, or for example, make -j3.

The install instructions suggest using make -j for parallel builds but
if this command is executed without a limit being provided to the -j
option, it may lock up the machine it is being run on. For those not
familiar with the -j option it is a bit of a trap. Changing the example
command to show the use of the limit should prevent people who are
building this repo for the first time and unfamiliar with the -j option
from copy-pasting the command verbatim and making that mistake.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v5.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document